In, for example, a base station device in a mobile communication system adopting the wideband code division multiple access (W-CDMA) method as a mobile communication method, since it is required to have a radio signal reach the remote mobile communication device, it is required to significantly amplify the signal by a power amplifier. In general, the output of the power amplifier linearly varies in an area with low input, but with the input exceeding a certain input level, the output becomes nonlinear, and goes into saturation. Since the power amplifier is typically used at an operating point near to the saturation point in order for improving the power efficiency, nonlinear distortion is caused by the nonlinearity of the amplifier.
Although the transmitter signal before amplification is suppressed in a low level since the signal component in other signal bands than the desired signal band is removed by a bandlimiting filter, a signal component of the signal after passing through the power amplifier leaks to the other signal bands (adjacent channels) than the desired signal band because of the generated nonlinear distortion. In, for example, the base station device, because of the high transmission power as described above, energy of the leakage electric power to the adjacent channels described above is strictly regulated, and accordingly, how to reduce such leakage electric power to the adjacent channels has become a major issue.
A predistortion method can be cited as one of distortion correction methods for correcting such nonlinear distortion of the power amplifier, and is becoming mainstream displacing the feedforward method because the power efficiency is emphasized in recent years.
The predistortion method is a method of correcting the distortion caused in the power amplifier by previously providing the input signal to the amplifier with the inverse characteristic of the AM-AM conversion and the AM-PM conversion, which are the nonlinear characteristics of the power amplifier.
FIG. 6 shows a configuration example of a functional block of an amplifier with a predistorter, which is composed of a power amplifier using the predistortion method.
An example of an operation performed by the amplifier with predistorter according to the present embodiment will be described.
An input signal to the amplifier with predistorter is input to an electric power detection section 71 and a predistortion section 73. The electric power detection section 71 detects the electric power (or alternatively the amplitude) of the input signal, and outputs the detection result to a distortion correction table 72 as a reference argument.
The distortion correction table 72 is configured as a look-up table (LUT) using, for example, a memory, and stores values for performing the distortion correction by the predistortion method, corresponding to the detection result of the electric power detection section 71 as the reference argument (address). Specifically, the distortion correction table 72 stores values of the AM-AM characteristic regarding the amplitude and the AM-PM characteristic regarding the phase generally taking the power or the amplitude of the input signal as an index, which are the inverse characteristics of the nonlinear characteristic of the amplifier (an amplifying section 74) to be the correction target.
The distortion table 72 outputs the value, which corresponds to the detected result input from the electric power detection section 71, to the predistortion section 73. The predistortion section 73 corrects the amplitude and the phase of the input signal in accordance with the value of the reference result input from the distortion correction table 72, and outputs the signal after thus corrected to the amplifying section 74.
The amplifying section 74 is composed of a power amplifier, and amplifies and then outputs the signal input from the predistortion section 73. The output signal is then output from the amplifier with the predistorter. It should be noted here that the signal input to the amplifying section 74 is previously provided with the distortion (amplitude distortion or phase distortion) corresponding to the inverse characteristic of the distortion characteristic of the power amplifier in the predistortion method, and by canceling this distortion and the distortion caused in the power amplifier each other, the output signal becomes a signal without distortion.
In order for accommodating the temperature variation, the secular variation, and so on, a control section 75 updates the storage content of the distortion correction table 72 based on input signals and output signals of the amplifier with the predistorter.
FIG. 7 shows a configuration example of the predistorter for performing digital processing.
An example of an operation performed by the predistorter according to the present embodiment will be described.
The input signal to the predistorter is input to an envelope detector 81 and a complex multiplier 83. The envelope detector 81 calculates sqrt(I2+Q2) to an in-phase (I) component “I” and a quadrature-phase (Q) component “Q” of the input signal for every sample to detect the instantaneous power (corresponding to the envelope in the RF band), and outputs the detection result to the LUT 82.
The LUT 82 is a distortion correction table composed of a memory or the like, and stores the values for controlling the distortion correction in the form of a complex vector in correspondence with the detection results by the envelope detector 81 as the reference argument (address), in the present embodiment. The LUT 82 outputs the corresponding complex vector for the distortion correction to the complex multiplier 83 using the detection result by the envelope detector 81 as the argument (address) of the table.
The complex multiplier 83 performs the complex multiplication on the input signal and the complex vector input from the LUT 82 and outputs them. Thus, the predistortion process is performed, and the output signal is output from the predistorter. It is common that digital predistorters operate with a sampling frequency several through several tens times as high as the transmission signal band width in order for correcting third, fifth, or higher order of mutual modulation distortion.
Patent Document 1: JP-A-2001-189685
Patent Document 2: JP-A-2004-040564
Patent Document 3: JP-A-2005-217690